Hydrocarbon solutions of resin esters of rosin-phenol condensates are known in the ink industry as solution phenolics. Solution phenolics are commonly employed by ink manufacturers in formulating vehicles for gravure printing inks. However, a major problem exists with the use of solution phenolics in that they tend to increase in viscosity over time. The rate of this viscosity growth is concentration-dependent, thereby worsening as the level of resin solids contained in a solution increases (see Table I below). Compounding this problem is the fact that viscosity growth in solution phenolics is also accompanied by undesirable variations in dilution values. As these time-dependent differences in the physical properties of solution phenolics cannot be predicted easily, the resulting variabilities create major problems for ink manufacturers.
These twin problems of viscosity growth and dilution value variation are graphically shown by the data listed in Table I below. Here, varying amounts of JONREZ.RTM. RP-346 (a rosin-phenolic resin manufactured by Westvaco, Inc.) were dissolved in toluene to produce solutions having differing resin solid levels. These solution phenolics were sealed in jars to prevent toluene evaporation and held at ambient temperatures (about 25.degree. C.) for 2 weeks.
TABLE I ______________________________________ Viscosity Growth as a Function of Time and Solids For Untreated Solution Phenolics Percent Physical Percent Solids.sup.a Property Day 1 Day 7 Day 14 Change ______________________________________ 45 Brookfield 16,800 41,000 88,530 427 Viscosity (cP) Dilution.sup.b 297 297 314 6 (mL) 37 Brookfield 1100 1950 3050 177 Viscosity (cP) Dilution 198 213 198 0 30 Brookfield 190 205 336 77 Viscosity (cP) Dilution.sup.b 130 144 140 8 ______________________________________ .sup.a Nominal percent solids. Actual solids ranged less than .+-. 0.5 units. .sup.b Dilution is reported as the milliliters of toluene added to 100 grams of RP346 solution (at indicated initial solids level) to lower its viscosity to 18 seconds in a #2 Shell cup at roughly 25.degree. C.
Table I shows that the rate of viscosity growth was proportional to the percent solids of the aged solution: the higher the solids level, the greater the increase in viscosity over time. For example, at 45% nominal solids a toluene solution of JONREZ RP-346 increased 427% (from 16,800 cP to 88,530 cP) in 14 days. At 37.5% nominal solids, a toluene solution of JONREZ RP-346 increased 177% in viscosity (from 1100 cP to 3050 cP) in 14 days. At 30% solids, a toluene solution of JONREZ RP-346 only increased 77% in viscosity in 14 days.
Heretofore there has been no solution to this problem; thus the ink industry is currently limited to retarding viscosity growth by utilizing rosin-phenolic resin solutions containing low solids concentrations (i.e., below 25% solids). The ability to create rosin-phenolic resin solutions which are viscosity stable at high solids levels would be greatly advantageous to the industry, especially in the areas of solution formulation, transport, and storage.
Therefore, it is the object of this invention to produce viscosity-stable high solids rosin-phenolic resin solutions for use in the formulation of vehicles for gravure printing inks.